Modularized Buck-Boost + Cuk Converter for High Voltage Series Connected Battery Cells Xi Lu, Wei Qian, and Fang Zheng Peng Department of Electrical and Computer Engineering Michigan State University East Lansing, MI USA [email protected]
Abstract—This paper proposes a new bidirectional dc-dc converter—buck-boost + Cuk converter—for series connected battery cells. Traditionally, in order to balance n battery cells in a string, (2n-1) switches are required by either the buck-boost converter or the Cuk converter. However, by nicely combine the buck-boost converter and the Cuk converter together, the proposed buck-boost + Cuk converter only requires n switches. It achieves almost half of the switch count reduction, without losing the advantage of modularization or sacrificing the device voltage stress, unlike many other existing one-switch-per-cell topologies. Meanwhile, it still maintains the buck-boost battery charge equalizer’s best characteristic—simple pulse width modulation (PWM)—50% duty cycle. Moreover, it offers a solution to bypass the open-circuit faulty cells, and prolong the life time of the whole battery string. Simulation and experimental results are given to demonstrate the functionality of the proposed buck-boost + Cuk converter. Index Terms—Buck-boost converter, cell charge equalization, Cuk converter, open circuit faulty bypass, minimum component count, modularization.
Cell voltages of a single rechargeable battery, such as NiCd, Lead-acid, NiMH, NiZn, and Lithium-ion, that are available in the nowadays market, range from 1.2 V to 3.6 V, due to their own chemical characteristics. However, there are many applications, which need a relatively high voltage battery pack (several tens to hundreds of volts), such as hybrid electric vehicles, uninterruptible power supply and so on. This requires a large number of battery cells to series together. As the number of series cells increases, close attention should be paid. For instance, internal resistance, state of charge, total capacity, internal temperature and degradation level, those parameters are gradually deviated as the increasing number of the repeated cycles of charge and discharge. As the charging current being the same for the whole series branch, due to the nonuniformities, some of the cells are overcharged, while some others are undercharged. In this case, the undercharged cell cannot be fully utilized. Moreover, the overcharge
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problem is the key killer for battery cells. Once one of the cells in the series is open circuit, the whole string is disconnected and cannot output any power at all. Consequently, equalization and faulty bypass are both very essential for the sake of extending the lifetime of the series battery cells. Therefore, a reliable battery equalizer is indeed needed to balance the cells during the charge/discharge process [1-3]. First of all, the requirements needed for a qualified battery equalizer are shown as follows: • • • • • •
Modularization ; Size and cost issues ; High equalization efficiency ; Device voltage stress; Fast equalization speed; Controller simplicity.
Therefore, a minimum component count, high efficiency, modularized battery equalizer is highly desired for the intelligent battery management system nowadays. Many researchers have been conducted on this topic, but rarely have any result can reach all the requirements stated above. Most of the approaches are shown as follows:
1. 2. 3. 4. 5. 6. 7. 8. 9.
Dissipative type, shown in Figure 1 (a); The switch capacitor type [5, 7-13], including both single-tiered and double tiered type; Buck-boost converter type , shown in Figure 1 (b). Cuk converter type [15-20], shown in Figure 1 (c); Resonant converter type [21, 22]; Transformer based type [23-28]; Magnetic coupled inductor type [29-33]; Other modified buck-boost type ; Multiphase chopper [35-38].
Figure 3 The proposed nondissipative battery equalizer: buck-boost +Cuk converter (a)
In this paper, the buck-boost + Cuk converter is proposed for the battery cell balancing application, especially suitable for high voltage battery packs, due to its modularization advantage and relatively low device voltage stress. It is named because the initial idea of this topology is coming from combining the two basic dc-dc converter together. This new converter has minimum switch count, with one switch per cell, leading to high efficiency. Meanwhile, it is modularized, for the convenience of adding and removing cells from the string. By possessing these two key advantages, this converter have more benefits, such that it still keeps the buck-boost/Cuk battery equalizer’s best characteristic: simple PWM method— 50% duty cycle. Simulation is given to demonstrate the functionality of the proposed buck-boost + Cuk battery equalizer.
Figure 1 Selected approaches for battery equalization: (a) dissipative battery equalizers: resistive shunt; resistive shunt with an active switch; zener diode shunt. (b) buck-boost converter; (c) nondissipative battery equalizers with Cuk converter; (d) nondissipative battery equalizers with CSC converter.
Figure 2 Nondissipative battery equalizers (minimum switch count): (a) buck-boost converter; (b) multiphase chopper; (c) magnetic coupled inductor.
The dissipative type is undesirable, because not only the energy is wasted, but also the extra heat resulted from the dissipated power raise a potential temperature problem too. Considering the other types, they either have too many switches (Type 2-5), more than one switch per cell, or they are not modularized due to the transformer or coupled inductor (Type 6-7), which is a killer for large number of series connected batteries. Multiphase chopper type seems to satisfy with all the requirements, however, its own PWM method leads to a high voltage stress, which is equal to the voltage of the whole battery stack, for all the switches. This limits this topology only working for small number of battery cell string.
PROPOSED BUCK-BOOST + CUK CONVERTER AND TOPOLOGY DERIVATION
In the application of battery equalizer, the bidirectional dcdc converter requires a negative output voltage instead of positive output voltage. Considering the modularization function, buck-boost converter, Cuk converter and CSC converter are suitable, shown in Figure 1 (b), (c) and (d). Figure 1 (b) has been discussed in , and Figure 1 (c) has been discussed in , while only Figure 1 (d) hasn’t been discussed anywhere yet. This is probably because, comparing with Figure 1 (b), the same function is accomplished, but one more capacitor is added to every battery equalizer module, which increase the passive component count. No matter what, all of them need too many switches (2n-2 switches, where n is the number of battery cells). Considering the minimum switch count, Figure 2 shows three topologies that are satisfied. Figure 2 (a) shows the buckboost converter. However, comparing to Figure 1 (b), this topology cannot transfer energy between the second and the third battery cells. This is because the middle wire decouples the top buck-boost converter from the bottom one. Therefore, in order to make the possible energy transfer between the second and the third cell, multiphase chopper in Figure 2 (b) is proposed by adding only one inductor in the middle [37, 38]. However, this topology requires a PWM control method, which results in a high voltage stress for every switch—the whole battery string’s voltage. This is impossible for a high voltage battery, such as several hundreds of volts. Figure 2 (c) shows the magnetic coupled inductor type. However, this is obviously not modularized. As more and more cells are added to the series, the coupled inductor has to be redesigned every
time. At the same time, it requires difficult control strategies as well [29, 30]. As a result, the buck-boost + Cuk converter is proposed to solve those problems stated above, taking a 4-cell battery as an example shown in Figure 3. Compared with the traditional buck-boost converter shown in Figure 1 (b), this new topology eliminates the buck-boost converters on the left hand side of the battery string (eliminate two switches and one inductor), and adds only one capacitor on the right hand side to transfer energy from the top two cells to the bottom two cells. By adding this energy transfer capacitor, the converter in the middle actually becomes a Cuk converter, which explains the principles. In fact, it is a very similar approach as Figure 2 (b), while an energy transfer capacitor is used instead of an inductor. However, the new topology solves the high voltage stress problem lies inherently in the multiphase chopper shown in Figure 2 (b), which enables the capability to work for high voltage batteries. III.
Figure 4 The circuit configuration of the proposed buck-boost + Cuk converter for 6 cell battery equalization.
OPERATING PRINCIPLES OF THE PROPOSED BUCKBOOST + CUK CONVERTER
The proposed battery equalizer is modularized. Figure 3 shows the circuit configuration for 4 cells and Figure 4 shows the case for 6 cells, indicating its modularized characteristic. The red part can be understood as modular A, while the blue part can be thought as modular B. If individual cell adding and removing is required, module A or module B can be added independently with the single cell, depending on the circumstances. If 2 cells are allowed to be added or removed at the same time, the blue part combined with the red part can be seen as one module. Observing from the topology, it can be seen that the equalizer only need n switches (one switch per cell), n/2 inductors, and (n-2)/2 capacitors. Knowing this topology is able to expand to any length, without the challenging of the whole battery pack’s voltage. The following analysis will only focus on a 4-cell battery string as an example. Obviously, the battery cells 1 & 2 are able to be balanced using the upper traditional buck-boost converter by controlling S1 & S2 with 50% duty cycle, the same to the cells 3 & 4 by controlling S3 & S4 with 50% duty cycle. The battery cells 2 & 3 are balanced using the Cuk converter in the middle by controlling S2 & S3 with 50% duty cycle. Therefore, this converter simply uses 50% duty cycle for every switch, and S1 is complimentary with S2, while S2 is complimentary with S3, and S3 is complimentary with S4. Right now, not considering the dead time and assuming V1>V2 and V3> V4, there are 2 switching mode, shown in Figure 5. A. Switching State 1 In this switching state, S1 and S3 are on, while S2 and S4 are off. The top inductor L1 will have the battery cell 1’s voltage, V1 across it during this period of time, while the bottom inductor L2 will have the battery cell 3’s voltage V3 across it. Hence, energy transfer capacitor C will have the voltage equal to V1+V2. During this period of time, battery cell 1 is discharging the
Figure 5 The switching states of the proposed buck-boost + Cuk converter (a) switching state 1; (b) switching state 2.
energy to L1 through S1. Battery cell 2 is being charged by the energy transfer capacitor C through L1 and S3. Battery cell 3 is in the same situation as battery cell 1, and is discharging the energy to L2. Finally, the battery cell 4 is doing nothing since S4 is turned off, and D4 is reverser biased as well. B. Switching State 2 In this switching state, S2 and S4 are on, while S1 and S3 are off. The top inductor L1 has to keep the original current direction, and its voltage is equal to the battery cell 2’s voltage. The bottom inductor L2’s voltage is equal to battery cell 4’s voltage. For the energy transfer capacitor C, the voltage across it is equal to V3+V4. During this period of time, battery cell 2 is being charged by L1 through S2. Battery cell 3 is discharging to C through S2 and L2. Battery cell 4 is being charged by L2 through S4. It can be seen that battery cell 1 and 3 are discharging all the time, while cell 2 and 4 are being charged all the time. It satisfy the assumption mentioned previously, which is V1>V2 and V3>V4. Have to mention that the inductor currents’ directions in Figure 5 are determined by the battery cell voltages, due to the intrinsic characteristic of the circuit, which means no need of outside control at all.
SIMULATION OF BATTERY EQUALIZATION
In order to simulate the proposed circuit, a battery model has to be chosen first, as shown in Figure 6 (a). A voltage source in series with an internal resistance is used. In order to have those battery cell voltages finally merge together, a capacitor paralleled with a resistor is in series with them. The simulation result for balancing function is shown in Figure 6. It can be seen that the 4 cells begin with different initial voltages, and after a period of time, their voltages are equalized by the buck-boost + Cuk converter. The detailed waveforms of the converter working principles are shown in Figure 7. V.
OPEN CIRCUIT BYPASS FUNCTION AND EXPERIMENTAL RESULTS
Usually after a long time operation, the battery cells are tended to be worn out. When this happens to the lead-acid battery, they are usually shown as an open circuit. If one of the cells in the whole string is open circuit, the whole battery string will not be able output any voltage at all. However, by using this battery equalizer, the open circuit cell can be bypassed by adding a capacitor across each battery cell. Actually this is the case for almost every battery equalizer, since this additional capacitor can absorb the switching current ripple, and give the battery cell a more smooth current which helps increasing the battery cell’s lifetime. Because of this capacitor, the voltage across the open circuit cell can be sustained to be close to the voltage of its neighbor cells.
Figure 6 (a) Battery model in SABER simulation; (b) Simulation results for the 4 different voltage cells merged together eventually.
Simulations for faulty bypass function are given under the worst case, such as the battery cell 2, 3, and 4 are all open circuit shown in Figure 8. With the only one cell left, it still can supply energy to the outside, pretended to be a 4 cell battery string. Simulation results are shown in Figure 9 and Figure 10. They indicate that, for different load condition, if only based on the 50% duty cycle control without any close-loop feedback control, both the heavy load and light load condition can balance the 4 cells very well. The lighter the load is, the better equalization result will be.
Figure 7 Simulation results for the battery equalizer (a) the energy transfer capacitor current; (b) the 2 inductor currents; (c) the energy transfer capacitor voltage; (d) 4 battery cells’ voltages.
Load :10 Ω
Experimental results for the bypass function are shown in Figure 11, Figure 12, and Figure 13 respectively. Figure 11 shows the output voltage, output current experimental results with and without the proposed buck-boost + Cuk converter. When the buck-boost + Cuk converter is not working, if 3 cells are open circuit, the output current and output voltages are almost zero. (Please note that the experimental result of the output voltage of not being zero is due to the setup shown in Figure 8, where the load resistor and those capacitors becomes a voltage divider, as the only left cell being the source.) After the buck-boost + Cuk converter starts to operate, the output voltage and output current are both become as normal as it seems no bad cells in the string. COMPARISON AMONG DIFFERENT BATTERY EQUALIZERS
The details are shown in Table 2.
Figure 8 Experiment setup for testing the open circuit bypass function of the proposed buck-boost +Cuk converter: 3 cells open circuit.
Figure 9 Simulation results when the load is heavy, R=5 Ω.
Figure 10 Simulation results when the load is light, R=200 Ω.
I L1 (1A/div)
I L1 (1A/div)
Figure 11 Experiment result for the bypass function. (a) without the proposed converter; (b) with the proposed Buck-Boost + Cuk converter
V1 (2.5V/div) V2 (2.5V/div)
I L2 (0.2 A/div)
Figure 12 Experiment result for the bypass function with Vc and IL2.
Figure 13 Experiment result for the bypass function for the 4 cells voltages
Number of switches Number of Inductors Number of Capacitors Voltage stress Control simplicity Equalization Speed Equalization Efficiency Size and Cost Modularization
COMPARISONS OF SEVERAL EXISTING BATTERY EQUALIZERS
2N-2 N-1 0 2 Vbatt Easy Fast High Large Yes
2N-2 N-1 N-1 2 Vbatt Easy Fast High Large Yes
Buck-Boost w/ Coupled Inductor N N/2 0 2 Vbatt Difficult Medium High Middle No
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Buck-Boost + Cuk
N N-1 0 n Vbatt Difficult Slow High Middle Sort of Yes
2N 0 2N-1 Vbatt Easy Slow Low Small Yes
N N/2 N/2-1 2 Vbatt Easy Fast High Middle Yes
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